Move mock aperture scatterguard logic into the mock device

Currently there is quite a bit of logic in `tests/devices.conftest` that is used to give a vaguely usable aperture_scatterguard device. Similar logic also exists in `mx-bluesky`. Following https://github.com/bluesky/ophyd-async/issues/1039 we can now make this logic the default for the device and remove it from the tests of `dodal`/`mx-bluesky`.

## Acceptance Criteria
* When you create a mock `ApertureScatterguard` you get something like the one created in the `conftest`
* The logic for this is no longer added in both the `dodal` and `mx-bluesky` tests